1. Field of the Invention
The present invention relates to a process for producing a magnetic metal powder suitable for magnetic recording.
2. Description of Related Art
Recently, a magnetic recording medium is increasingly required to be improved in recording density, to be smaller in size and to be higher in performance. Thus, attention has been paid to an iron or iron-based alloy powder (hereinafter referred to as "magnetic metal powder") which is higher in saturation magnetization and coercivity than a magnetic iron oxide powder as a magnetic powder for magnetic recording. The magnetic metal powder is now being put into practical use for digital audio tapes or 8-mm video tapes. Recently, it has been expected to apply the magnetic metal powder to high performance recording mediums such as high image quality video tapes and high recording density disks.
Recently, magnetic metal powders for high performance magnetic recording, particularly when the powders are comprised of, for example, acicular particles, are required to be so fine that the particles have a major axis of less than about 0.5 .mu.m, particularly, less than about 0.3 .mu.m and contain no sintered particles. In addition, in order to attain the enhancement of S/N ratio of recording medium, finer particles are required. For this purpose, it has become further important to reduce the size of crystallites (hereinafter referred to as "Lc") which constitute the acicular particles as well as to reduce the size of the acicular particles. Furthermore, when such finer powders are used as a magnetic paint, they are required to have further superiority in dispersibility, orientability, packing property and surface smoothness of coatings, when coated. However, magnetic powders are apt to be damaged in dispersibility and packing property as the powders are made finer. Therefore, in order to make the magnetic iron metal powder finer without damaging the dispersibility and packing property of the powder, it is desired to use hydrous iron oxides or iron oxides in a fine form and of a good particle size distribution as the starting material. However, the finer the starting material, the greater a tendency to make a change in the acicular form owing to intraparticle sintering, or to cause crosslinking or growing of particles owing to interparticle sintering during the reduction for obtaining the final products, which considerably has an adverse effect on the magnetic characteristics of the products.
Hitherto, various methods have been proposed to solve the problems as mentioned above. Of them, a method has usually been carried out in which various deformation-preventing agents are adhered by coating or mixing onto the surface of the hydrous iron oxides or iron oxides and then these oxides are subjected to the heat treatment for reduction. As the deformation-preventing agent have been proposed to use a silicon compound, aluminum compound, a combination of the aluminum compound and the silicon compound, or a boron compound (see Japanese Patent KOKOKU (Post-Exam. Publn.) Nos. Sho 54-42832, Sho 59-32881, Sho 59-19964 and Hei 1-22968 and Japanese KOKAI (Laid-Open) Nos. Sho 57-106526, Sho 58-48611, Sho 58-48612, Sho 59-5603, Sho 61-88505, Sho 61-174304 and Sho 64-84601). It has also been proposed to subject the particles to a tempering at high-temperature or a heat treatment in a steam containing atmosphere prior to the reduction to obtain magnetic metal powders, or subject the particles reduced to magnetite to the heat treatment, thereby retaining the acicular form of the crystals in the starting material or inhibiting the sintering of particles. However, there are still many problems which have to be solved, although the proposed methods can inhibit the deformation of the particle form and the interparticle sintering to some extent: such problems are that the progress of the reduction is easily hindered; the treatments are complicated; costs are increased; the amount of the form-retaining agent to be coated or deposited is variable depending upon the treating conditions, thereby causing variation in the magnetic characteristics of the resulting magnetic metal powders; or the shape of particles as substrate is broken or the particles are ruptured in the course of the deposition of the deformation-preventing agent by mixing with the particles.